Multiple tools to find and track Listeria are proving successful, even though the rare pathogen was responsible last year for the most deadly outbreak of foodborne illness in decades.


Dr. Barbara Mahon from the federal Centers for Disease Control and Prevention Tuesday opened the 40th Annual Rocky Mountain Food Safety Conference in Golden, CO by putting last year’s deadly Listeria outbreak into some context, describing new efforts to control the strange bacteria.

Mahon said Listeria is a highly virulent pathogen that annually causes 1,600 illnesses and 260 deaths. Last year’s Listeria outbreak traced back to Colorado-grown cantaloupe caused 146 illnesses and at least 32 deaths.

Listeria’s threat is best illustrated by the fact that it accounts for less than one percent of all foodborne illnesses, but causes almost 20 percent of all food illness deaths. Killing Listeria, however, is not easy. The bacteria continue to thrive when refrigerated. 

Finding Listeria on whole cantaloupes was new. The dangerous foodborne bacterium is usually associated with hot dogs, deli meats and soft, Mexican-style cheeses.

Mahon said the 95 percent hospitalization rate from listeriosis, and its fatality rate of at least 16 percent, makes it a very dangerous illness for the populations it impacts the most. Those include pregnant women, newborn infants and the elderly or anyone with a compromised immune system.

CDC’s “wake up call” for Listeria came in 1985 with an outbreak in Los Angles County, CA caused by Mexican cheese that involved mostly pregnant Hispanic women, Mahon said.   Before the epidemic was stemmed, there were 28 deaths and 20 stillbirths.

Mahon said four surveillance tools set up in the years that followed – FoodNet, PulseNet, Listeria Initiative, and the Food Disease Outbreak Surveillance System or FDOSS – have combined to improve the Listeria outlook.

FoodNet, set up in 1995, monitors trends for individual foodborne illnesses like E coli O157:H7, Salmonella and Listeria.  PulseNet connects cases to track outbreaks of foodborne illness.  Mahon says that while only the large multistate outbreaks get much attention, about 1,000 outbreaks a year are logged.  Most are single-state outbreaks.

The Listeria Initiative involved a 16-page questionnaire that CDC encourages state and local health departments to use to interview those infected by Listeria. The goal is to get the interview completed as quickly and thoroughly as possible.

Since it began in 2002, the number of states involved in the Listeria Initiative has grown to 42, up from 19.

Alicia Cronquist from the Colorado Department of Public Health and Environment said the Listeria initiative questionnaire, while lengthy, was instrumental in last year’s Listeria investigation.

FDOSS, which has been around since 1973, allows CDC to compare outbreak data over time.   Mahon says the four systems together allow CDC to make some conclusions.   

For example, during the 1990s, all efforts against Listeria brought the total incidence of infection down, from about 8 cases per million to about 2. It’s remained at about that level since.   

However, Mahon says the incidence of Listeria infection involving pregnant women is on the upswing, while the danger for elderly and populations with compromised immunity has been holding fairly steady.  

With its four systems now working in a more robust way, CDC is identifying more Listeria outbreaks. “The fact that we are seeing more outbreaks we view as a good news,” Mahon says. Just because no one knew about them in the past does not mean that outbreaks were not occurring earlier.

Mahon said deli meats, cheeses, and hot dogs remain prominent as the typical sources of Listeria outbreaks.

The U.S. Food and Drug Administration’s (FDA’s) Devin Koontz updated the conference on the federal investigation into Jensen Farms, the cantaloupe grower responsible for last year’s Listeria outbreak.  

Koontz said beyond the existing theories – contamination of a cull truck that went between Jensen’s packing house and a cattle yard, contamination of equipment that included a used potato washer, or just low-level sporadic contamination – FDA cannot say with any certainty how the Listeria entered the operation.

The FDA official said Jensen Farms agreed to inspection before resuming any cantaloupe operations.  “We want to make sure that this outbreak does not fire back up,” Koontz said.

Agricultural sources in Colorado say Jensen Farms is not re-entering the cantaloupe business this year.